Recently, a digital versatile disk (DVD) has drawn attention as a high-capacity optical recording medium, since it is capable of recording digital information at a recording density approximately six times that of a compact disk (CD). Further, with an increase in information to be recorded, a higher-density optical recording medium is demanded. Here, to achieve higher densification than that of a DVD (a light source in a DVD system has a wavelength of 660 nm, and an objective lens therein has a numerical aperture (NA) of 0.6), it is necessary to shorten the wavelength of the light source and increase the NA of the objective lens. For example, a recording density five times that of the DVD can be achieved by using a blue laser with a wavelength of 405 nm and an objective lens with a NA of 0.85.
However, in an optical recording/reproducing device of a high-density optical recording medium using the foregoing blue laser, margins in recording and reproducing are extremely narrow, and hence, the occurrence of an aberration due to a tilt of an optical recording medium becomes a problem. To cope with this, JP 11(1999)-312327A has proposed an optical head that is configured to carry out recording and reproducing by tilting an objective lens so as to decrease an aberration that occurs according to an amount of a tilt through which an optical recording medium tilts (such an amount of a tilt of an optical recording medium hereinafter is referred to as a tilting amount).
Here, an example of the above-mentioned conventional optical head is described with reference to a drawing. FIG. 11 is a schematic view illustrating a configuration of an optical head in the prior art. In FIG. 11, 111 denotes a light source, and for example, a semiconductor laser element is used as the light source 111. The light source 111 emits coherent light for recording/reproducing with respect to a recording layer of an optical recording medium 116. 112 is a collimator lens, which is a lens for converting divergent light emitted from the light source 111 into parallel light. 113 denotes a beam splitter, which is an optical element for splitting light incident therein. 114 denotes a mirror, which is an optical element that reflects incident light and directs the same toward the optical recording medium 116. 115 denotes an objective lens, which is a lens for focusing light on a recording layer in the optical recording medium 116. 118 denotes a lens holding member for holding the objective lens 115, and the lens holding member 118 also functions as an objective lens tilting means for tilting the objective lens 115. 119 denotes a tilt sensor, which is intended to detect a tilting amount of the optical recording medium 116. 123 denotes a tilt detecting circuit, and 125 denotes a tilt controlling circuit. The operation for controlling a tilt of the objective lens 115 is performed by the tilt sensor 119, the tilt detecting circuit 123, the tilt controlling circuit 125, and the lens holding member 118 functioning as the objective lens tilting means, so as to eliminate a tilt of the objective lens 115 relative to the optical recording medium 116. Further, 117 denotes a detection optical system, 120 denotes a focus error signal detecting circuit, 121 denotes a tracking error signal detecting circuit, 122 denotes a reproduction signal detecting circuit, and 124 denotes a controller.
Next, an operation of the optical head having the foregoing configuration is described with reference to FIG. 11.
Linearly polarized light emitted from the light source 111 is converted into parallel light by the collimator lens 112. The light thus converted into parallel light passes through the beam splitter 113, is reflected by the mirror 114, and is focused on the recording layer of the optical recording medium 116 by the objective lens 115.
The light reflected by the optical recording medium 116 passes through the objective lens 115, and is reflected by the mirror 114 and the beam splitter 113 successively. Then, it is guided from the detection optical system 117 to the focus error signal detecting circuit 120, the tracking error signal detecting circuit 121, and the reproduction signal detecting circuit 122, where a focus error signal, a tracking error signal, and a reproduction signal are detected, respectively. The focus error signal and the tracking error signal are detected by a known technique, for instance, the astigmatism method, the push-pull method, etc. Offset amounts are added to the detected focus error signal and tracking error signal by the controller 124 as required.
A focus controlling means (not shown) controls the position of the objective lens 115 in its optical axis direction according to the focus error signal so that the light is focused on the recording layer of the optical recording medium 116 in a well-focused state at all times. Further, a tracking controlling means (not shown) controls the position of the objective lens 115 in a radial direction of the optical recording medium 116 according to the tracking error signal so that light is focused on a desired track on the optical recording medium 116.
A tilt of the objective lens 115 relative to the optical recording medium 116 is detected by the tilt detecting circuit 123 via the tilt sensor 119 provided beside the objective lens 115. The tilt controlling circuit 125 feeds the lens holding member 118 with a signal for tilting the objective lens 115 according to the tilt signal from the tilt detecting circuit 123. With this operation, the objective lens 115 is tilted so that the tilt thereof relative to the optical recording medium 116 is eliminated.
Even with a tilt of the optical recording medium 116 relative to the objective lens 115, the foregoing configuration ensures a decrease in an aberration by detecting the tilting amount and tilting the objective lens 115, thereby allowing stable recording and reproducing to be conducted.
However, the optical head with the foregoing configuration cannot be used for a multi-layer optical recording medium in which information is recorded at a density higher than that in a DVD. For instance, assume a case in which recording and reproducing are carried out with respect to a single-layer optical recording medium with a substrate thickness of 0.1 mm and a multi-layer optical recording medium having a first layer with a substrate thickness of 0.08 mm and a second layer with a substrate thickness of 0.12 mm, by using a system whose light source has a wavelength of 405 nm and whose objective lens has an NA of 0.85. Here, the objective lens is designed so as not to cause a spherical aberration with respect to a substrate thickness of the single-layer optical recording medium. FIG. 3 illustrates the relationship between a tilting amount of the optical recording medium and a coma occurring in each layer thereof, as to the single-layer optical recording medium and the multi-layer optical recording medium (two-layer recording medium). As shown in FIG. 3, even with the same tilting amount of the optical recording medium, the amount of a coma varies with the substrate thickness. This implies that even with the same tilting amount of the optical recording medium, the amounts of tilts through which the objective lens are tilted (such an amount of a tilt of the objective lens hereinafter is referred to as a tilted amount) in the case where coma is corrected by tilting the objective lens differ from one another for the respective layers. Therefore, a problem occurs in open loop control such that not an aberration that occurs due to a tilt of an optical recording medium but a tilt of an optical recording medium is detected, and an objective lens is tilted according to the tilt.